1. Field of the Invention
This invention relates to an image enlarging or contracting method for enlarging or contracting an image in a predetermined direction or in two directions by reading out a recorded image in order to obtain image signal components and carrying out interpolation on the image signal components.
2. Description of the Prior Art
Techniques for reading out a recorded image at its picture elements arrayed at predetermined intervals in two directions in order to obtain a plurality of image signal components which represent information about the image, and reproducing an image from the image signal components have heretofore been known in various fields. Also, there have heretofore been known techniques for reproducing an image enlarged or contracted in a predetermined direction by carrying out interpolation on the image signal components along every array of picture elements standing in the predetermined direction in order to obtain a number of interpolated image signal components, which is different from the number of the original image signal components, along every array of picture elements.
FIG. 4 shows how interpolation on image signal components has heretofore been carried out most popularly.
FIG. 4 shows an example wherein an image is enlarged to a size 4/3 times as large as the size of the original image in the horizontal direction. White dots indicate the center points of picture elements arrayed in the predetermined direction, i.e. in the direction of image enlargement or contraction, and a0, a1, a2, . . . denote the original image signal components detected at corresponding picture elements. Interpolation is carried out on the image signal components in order to obtain interpolated image signal components b0, b1, b2, . . . of picture elements having their center points at black dots shown at the lower part of FIG. 4.
By way of example, the interpolated image signal components b0, b1, b2, . . . are obtained by carrying out interpolation of first order expressed as ##EQU1##
The interpolated image signal components b0, b1, b2, . . . can also be obtained by carrying out interpolating operations of a high order.
When an image is reproduced from the "x" interpolated image signal components b0, b1, b2, . . . , an image enlarged in the predetermined direction as compared with the original image is obtained. An image contracted in a predetermined direction as compared with the original image can be obtained in the same manner.
In cases where interpolation on image signal components is carried out in accordance with the conventional techniques and an image enlarged or contracted in a predetermined direction is reproduced on a display means, such as a CRT display device, from the interpolated image signal components, a moire fringe-shaped artifact arises in the reproduced image, and renders the reproduced image unsuitable for viewing purposes.
Also, there have heretofore been known techniques for reading out a recorded image in order to obtain a plurality of image signal components corresponding to picture elements of the image arrayed at predetermined intervals in two directions, reproducing an image enlarged or contracted in two directions by carrying out interpolation on the image signal components in order to obtain interpolated image signal components corresponding to a number of picture elements, which is different from the number of the original picture elements.
FIG. 5 shows how first-order interpolation on image signal components has heretofore been carried out most popularly in order to enlarge or contract an image in two directions. FIG. 5 shows an example wherein an image is contracted to a size 5/6 times as large as the size of the original image. The "o" marks indicate the center points of the original picture elements arrayed at predetermined intervals in two directions on a recorded image. The center points of the original picture elements are represented by aij where i and j each denote an integer. The "x" marks indicate the center points of interpolated picture elements at which interpolated image signal components are to be obtained by interpolating operations. The center points of the interpolated picture elements are represented by bij where i and j each denote an integer. The image signal components corresponding to the center points aij and bij of the picture elements are also represented by aij and bij. Also, points on the image which are used midway during the interpolating operations and the image signal components corresponding to said points are represented by pi where i denotes an integer.
The distance l on the image shown in FIG. 5 is equal to the distance of six intervals as for aij, and is equal to the distance of five intervals as for bij. Therefore, when an image is reproduced and displayed from the interpolated image signal components bij on an image displaying device such as a CRT display device, the reproduced image is contracted to a size 5/6 times as large as the size of the image reproduced and displayed from the original image signal components aij.
How the interpolated image signal component b11 corresponding to a center point b11 is obtained by the interpolating operations will be described hereinbelow.
Interpolation of first order is carried out as described below:
(1) The value of an image signal component p1 corresponding to a point p1 is calculated by linear interpolation from the values of original image signal components a11 and a12.
(2) The value of an image signal component p2 corresponding to a point p2 is calculated by linear interpolation from the values of original image signal components a21 and a22.
(3) The value of an interpolated image signal component b11 corresponding to a center point b11 is calculated by linear interpolation from the values of the image signal components p1 and p2.
Operations (1), (2) and (3) are repeated for all center points bij in order to obtain the interpolated image signal components bij corresponding to the image contracted to a size 5/6 times as large as the size of the original image.
Also, in order to obtain, for example, the interpolated image signal component b11 corresponding to the center point b11, there has heretofore been known a technique wherein interpolation of second order is carried out by use of the original image signal components at the center points a01, a02, a03, a11, a12, a13, a21, a22 and a23. There has also been known a technique wherein an appropriate value among the values at the center points a11, a12, a21 and a22 located in the vicinity of the center point b11 (for example, the value of the point nearest to the center point b11 among the center points a11, a12, a21 and a22, or the maximum value or the minimum value among the values of the center points a11, a12, a21 and a22) is employed as the value of the interpolated image signal component b11.
In this specification, various techniques for obtaining values of arbitrary points in an image are referred to as the interpolation.
With the conventional interpolating operations described above with reference to FIG. 5, the original image signal components aij have predetermined sharpness, and the sharpness of the interpolated image signal components bij is fixed, depending on a mode of interpolating operations employed. In order to change the sharpness of the interpolated image signal components bij, it was necessary to carry out specific operation processing on the interpolated image signal components bij for changing the sharpness (hereinafter referred to as sharpness operations).
The term "sharpness operations" as used herein means, for example, the operations for averaging the image signal components at surrounding picture elements and blurring the image in cases where the sharpness is to be decreased.
When both the interpolating operations and the sharpness operations are carried out on the image signal components, a longer time is required to complete the operations, and the image processing speed cannot be increased. Also, in order to carry out the operations by a hardware means for the purpose of increasing the image processing speed, it is necessary to use both the apparatus for carrying out the interpolating operations and the apparatus for carrying out the sharpness operations.